German Interior Minister Horst Seehofer offered his resignation to party colleagues late on Sunday, party officials said, escalating a row over migration with Chancellor Angela Merkel that threatens her fragile government.

Seehofer said he was ready to step down as minister and as chair of his Christian Social Union (CSU) at a meeting where his party’s leadership was discussing whether to accept immigration proposals Merkel brought back from Brussels last week.

The move makes the future of Merkel’s government even more uncertain, since her Christian Democrats party (CDU) relies on the Bavarian conservative CSU to maintain power through a coalition formed three months ago to end a political vacuum.

Merkel lost votes to the far-right in elections last September, and she has been forced to turn to European Union neighbours to help resolve the row over how to deal with migrants trying to enter the country.

Germany’s political crisis is the latest sign of a growing divide across the EU between those who want to maintain open borders and those who want to restrict the number of migrants entering the bloc.

Seehofer, who has demanded that Merkel toughen her open-doors refugee policy, earlier told colleagues that in spite of the measures agreed with EU leaders, he saw no alternative to turning some migrants back at the border, a party source said.

Merkel rejects that idea.

CSU leaders, divided over how to face down a challenge from the anti-immigration Alternative for Germany (AfD) in October’s regional election, were trying to persuade Seehofer to change his mind about resigning, the officials said.

Seehofer told party colleagues at an executive committee meeting that discussions with Merkel had been fruitless, according to a party source.

But others in the CSU have pointed to opinion polls showing that Bavarians have more sympathy for Merkel than for either Seehofer or Bavarian Premier Markus Soeder.

By appealing to migration hardliners in the CSU, they argue, the party could lose votes in the centre.

Earlier this week, EU leaders hammered out a deal to share out refugees on a voluntary basis and create “controlled centres” inside the European Union to process asylum requests.

Merkel said in an interview with ZDF television that the formal agreements and verbal commitments she had secured from her EU partners would have the migration-stemming effect the CSU wanted to achieve, but in a more European-minded fashion.

She reiterated her determination to act in way that was “not unilateral” and that was “not to the detriment of third parties”.

“The sum of all we’ve agreed is equivalent to what the CSU wants – that’s my personal view, but the CSU must decide for themselves,” she said.

“It is also sustainable and in accordance with the European ideal. Europe is slow, and we aren’t yet where we want to be… In my view Europe will be held together, otherwise free movement could have been in danger,” she added.

A document circulated by Merkel to coalition allies on Friday night outlined repatriation accords with 16 countries and proposed reception centres in Germany where migrants would undergo an accelerated asylum procedure — steps that represent a significant hardening of her 2015 open-door asylum policy.

The Czech Republic, Poland and Hungary, whose Prime Minister Viktor Orban has long sought to position himself as Merkel’s nemesis in the immigration debate polarising the continent, later said they had signed no bilateral agreements.

In the interview, Merkel said she regretted any misunderstandings, but that she had been given “political commitments” and had not said any deals had been signed.

Volker Bouffier, premier of the state of Hessen and Merkel’s close ally, said that the CSU should be pleased at what it had achieved so far as he arrived at a similarly protracted meeting of his and Merkel’s CDU in Berlin.

“Europe has moved further than ever because of the CSU’s pressure,” he said.

While most analysts expect Merkel to survive the clash with the CSU, it is unlikely to be the last occasion on which the sister party seeks to distance itself from a chancellor it sees as too centrist for its own supporters.

But some in German political circles say that the row actually reflects Seehofer’s longstanding rivalry with Soeder, who unseated him as Bavarian premier, accusing them of holding Germany hostage to a Bavarian issue.

“Seehofer, who has almost totally lost power in the CSU, now appears to want to bring down the chancellor in a kind of indirect political suicide … ending his career under the motto: first me, then the party, and maybe then the country,” wrote the Hannoversche Allgemeine newspaper in an editorial.